Bistable storage device and method of operation utilizing a storage target exhibiting electrical breakdown

ABSTRACT

An information-handling device including: a storage target; electron gun means for producing on the storage target an electrostatic charge pattern embodying the desired information; suitable connection means for applying to and extracting from the storage target electrical signals; and a collector electrode. The storage target comprises a signal plate and a storage layer disposed thereon, the storage layer comprising an electrically insulating material and being adapted to exhibit electrical breakdown at a certain potential between the first and second cross-over potentials of the insulating material. A novel method of operating the device is also disclosed.

United States Patent [191 Silver 1 51 June 5, 1973 [54] BISTABLE STORAGE DEVICE AND 3,293,474 12/1966 Gibson "315/12 x METHOD OF ()PERATION UTILIZING 3,277,333 10/1966 Williams ct a1 ..315 12 A STORAGE TARGET EXHIBITING ELECTRICAL BREAKDOWN [75] Inventor: Robert Steven Silver, Kendall Park,

[73] Assignee: RCA Corporation, New York, NY.

22 Filed: Feb. 2, 1-970 [21] Appl. No.: 7,833

[52] U.S. Cl ..3l5/12, 313/68 D [51] Int. Cl ..H0lj 29/41 [58] Field of Search ..315/1012; 313/89, 65, 66,

[56] References Cited UNlTED STATES PATENTS 3,631,294 12/1971 Hofstein ..315/l0 2,859,376 11/1958 Kirkpatrick ..3l5/l2 X 2,912,615 11/1959 Lubszynski ..313/89 X 3,428,850 2/1969 Crowell et a1. ..3l5/12 Primary Examiner-Benjamin R. Padgett Assistant Examiner-J. M. Potenza Att0rneyGlenn H. Bruestle [57] ABSTRACT An information-handling device including: a storage target; electron gun means for producing on the storage target an electrostatic charge pattern embodying the desired information; suitable connection means for applying to and extracting from the storage target electrical signals; and a collector electrode.

The storage target comprises a signal plate and a storage layer disposed thereon, the storage layer comprising an electrically insulating material and being adapted to exhibit electrical breakdown at a certain potential between the first and second cross-over potentials of the insulating material.

A novel method of operating the device is also dis closed.

6 Claims, 13 Drawing Figures Pg 51 3O% I6 20 1S4, 1 o H,

PATENIE m 5197s SHEET 1 UP 2 I N VEN TOR. Robert S. Sg'lver A TTORNE Y BISTABLE STORAGE DEVICE AND METHOD OF OPERATION UTILIZING A STORAGE TARGET EXHIBITING ELECTRICAL BREAKDOWN BACKGROUND OF THE INVENTION This invention relates to a novel information-storage device and to a novel method of operation thereof.

Some previous storage devices include a storage target made of an insulating body mounted on a conductive signal plate, the insulating body usually being perforated such that parts of the underlying substrate are accessible to electrons produced within the device and directed toward the target. The operation of such a device, is known in the art. Briefly, an electrostatic charge pattern is'provided on the insulating body, usually by primary electron impingement on the target causing secondary electrons to be selectively emitted therefrom so that negative electrostatic potentials are created at various portions of the insulating body. The charge pattern represents the information that is sought to be stored, the distribution of the electrostatic charges being in accordance with such information. Thereafter, an electron beam is caused to scan the target in raster fashion, the impingement of the beam on the accessible areas of the substrate being modulated by the electrostatic charges (and, therefore, the electrostatic potential) at the areas of the storage body in proximity therewith. The more negatively charged areas of the storage body hinder or completely prohibit the landing of beam electrons on the substrate, whereas less negatively charged areas of the storage body and those having a zero electrostatic charge potential allow more or substantially all of the beam electrons to land on the substrate. Those beam electrons that do land on the substrate produce a signal thereon. These signals are a read out of the information on the storage body, the signals being utilized in known fashion (e.g., to reproduce the information as a visible image on a display tube). Such a device and mode of operation as that just described is not fully satisfactory because, inter alia, the level and/or distribution of the electrostatic charges on the storage body can be altered as the information is read out several times or after a continuous read-out of several hours. This results in the partial erasure or total elimination of the information that is stored and, consequently, poor resolution and an incorrect readout of the information, or no information at all, respectively.

SUMMARY OF THE INVENTION The present invention relates to a novel information storage device and to a novel method for operating the device. The device includes: an information storage structure comprising storage regions and electrically conducting or semiconducting regions, which storage regions comprise an electrically insulating material and are adapted to exhibit electrical breakdown at a certain electrical potential thereacross; means for producing on these storage regions an electrostatic charge pattern that embodies the information, which charge pattern includes the abovementioned certain electrical potential at at least one of the storage regions so as to produce such electrical breakdown at the at least one of the storage regions; and means for reading the information on the storage regions via the electrically conducting or semiconducting regions.

In a preferred embodiment, the device includes: an evacuated envelope that contains a storage target comprising an electrically conductive or semiconductive substrate having a major surface and a storage layer disposed on the substrate, electron gun means for writing information on the storage target in the form of an electrostatic charge pattern, means for reading the information, and means for erasing the information. The storage layer comprises an electrically insulating material that exhibits a secondary electron emission ratio greater than unity within a certain potential range, and the storage layer is adapted to break down electrically at at least one potential level within this range.

The storage layer may be a continuous layer; an apertured layer; or a plurality of discrete bodies that are electrically independent of each other.

The electrostatic charge pattern embodying the in formation comprises substantially two (bistable) potential levels: zero volts at the unwritten regions of the storage layer; and a potential value equal to the breakdown voltage and between the first and second crossover potentials. The information is read out in a bistable form, the novel device allowing the continuous read-out of information for comparatively long periods of time without any significant impairment or distortion of the information.

A method of operating the device includes producing a charge pattern on the storage target, the pattern com prising bistable potential levels, and reading out the information by causing secondary electron emission from the written areas (which are at electrical breakdown potential) of the storage layer. This secondary electron emission results in electrical charge carriers being transferred between the signal plate of the target and the storage layer thereof, such transfer of charge carriers producing an electrical signal that is extracted from the target and utilized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of one embodiment of the novel storage tube.

FIGS. 2, 3, and 4 are fragmentary perspective views, partly in section, of other embodiments of storage targets that may be employed in the novel storage tube.

FIGS. 5 through 11, inclusive, are schematic transverse sectional views through the target of FIG. 1 to explain various phases of a mode of operating one embodiment of the novel storage tube.

FIGS. 12 and 13 are schematic sectional transverse views through the target to explain certain operational aspects of the novel storage tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates an information storage tube 10 which incorporates the invention disclosed herein. The storage tube 10 includes an evacuated envelope 12, which may be of any suitable material, such as glass. Within the envelope I2 is an electron gun 14 including a cathode 16, a control electrode 18, and an accelerating electrode 22. A storage target, or storage structure, 20 is disposed in the envelope l2 opposite the electron gun 14. The accelerating electrode 22 is electrically connected by a lead 23 to the positive side of a D.C. potential source 24 and the control electrode 18 is connected by a lead 26 to a source 28 of an input signal whose wave form is to be stored on the target 20. The

storage target 20 is schematically shown in FIG. 1 and specific target structures are disclosed in subsequent figures herein.

In a preferred embodiment shown in FIG. 2, the storage target 20 comprises an electrically conducting or semiconducting substrate 30 99 having disposed on a major surface 32 thereof, storage regions in the form of a storage layer 34. For example, the substrate 30 consists essentially of a metallic (conductive) material or a semiconductor material (e.g., silicon or germanium) that is undoped or doped to a suitable level of conductivity. The polycrystalline or monocrystalline form of either germanium or silicon, for example, is satisfactory. The substrate 30 is sufficiently thick to provide a self-supporting target structure, for example mils. The storage layer 34 comprises an ordered, or system atic, array of discrete bodies, in the form of spaced lands 36, covering only a portion of the major surface 32, other portions of the major surface 32 being directly accessible via the spaces 38 between the land 36 of the storage layer 34. The storage layer 34 consists essentially of a secondary electron-emissive insulating material, such as, for example, magnesium, fluoride, aluminum oxide, or an insulating compound of a semiconductor material. The thickness of the storage layer 34 should be such that electrical breakdown between the exposed major surface 37 thereof and the underlying substrate 30 can occur at electrostatic charge potentials (which are located at the storage layer) between the first and second cross-over potentials for the particular material comprising the storage layer 34. Where the substrate 30 consists essentially of a particular type of semiconductor material, the storage layer 34 preferably consists of an insulating compound of such semiconductor material, for reasons explained below. The insulating compound is preferably the dioxide or nitride of either silicon or germanium. An electrostatic charge potential of about +60 volts is adequate for electrical breakdown in a silicon dioxide storage layer of about 1,000 A thickness, 460 volts also being between the first and second cross-over potentials therefore. As used with respect to the invention herein, the term layer includes a continuous layer; an apertured layer (e.g., one having a network configuration, as in FIG. 4); and a layer comprising an array of spaced discrete bodies (e.g., 36 in FIG. 2 and 74 in FIG. 3). A storage layer comprising discrete bodies of insulating material (FIGS. 2 and 3) is preferred because the-isolation of the various bodies from each other reduces the spreading of electrical charges over the storage layer by electrical charge dissipation or by the lateral electrical breakdown of the storage layers such spreading impairing image resolution.

Referring again to FIG. I, the substrate 30, which serves as the signal plate of the target 20, is provided with an electrical lead 40 whereby both a variable electrical potential from a potential source 42 can be applied to and an electrical output signal can be extracted from the substrate 30. The output signal may'be transmitted to, for example, a display tube 44 or used in another manner. The storage target is disposed within the storage tube 10 such that the storage layer 34 faces, and is substantially perpendicular to the axis of, the electron gun 14. The target 20 is supported in the storage tube 10 by a target support ring 46, or other means known in the art. Between the electron gun l4 and the storage target 20, there is disposed an electron permeable secondary electron collector electrode 48 supported on a collector support ring 50. The storage target 20 is electrically separated from the collector support ring 50 by insulating spacer ring 52. A lead 54 for applying an electrical potential from a D.C. potential source 56 is connected to the collector electrode 48. Disposed outside the envelope 12 are magnetic beamfocusing means 58 and magnetic beam-deflecting means 60, the deflecting means 60 being adapted to cause an electron beam 62 to scan the storage target 20 in raster fashion. Alternatively, electrostatic focusing and deflecting means (not shown) may be used.

Generally, in the operation of the storage tube 10 illustrated in FIG. 1, the electron gun 14 is employed to produce, at different times, a writing beam, a reading beam, or an erasing beam. The cathode 16 of the electron gun 14 is operated at ground potential. The substrate 30 of the storage target 20, acting as a signal plate, is operated at a DC. potential that is determined by the particular operation (viz., write, read, or erase) being carried out, that potential generally being positive with respect to the cathode 16. The focusing means 58 and deflection means 60, respectively, focus the beam 62 and deflect the beam 62 to scan the storage target 20 in raster fashion. In the writing operation, an electron charge image, or pattern, is formed on the storage layer 34 as a comparatively high velocity writing beam is scanned across the target 20. The electron charge image that is formed is a function of both the potential of substrate 30 and the beam current of the writing beam, the former determining the secondary electron emission ratio and the latter determining the number of primary electrons impinging upon and, hence, the number of secondary electrons being emitted by, the storage layer 34. The beam current is modulated by the control electrode 18 in accordance with the potential applied thereto. The control electrode potential is provided by an input signal transmitted to the tube 10 by connecting means 26. Variation of the input signal, and, hence, beam current, while the writing beam scans the storage layer 34, causes the various portions of the storage layer surface 37 that are impinged by the modulated beam to experience different beam currents. Where the substrate potential exceeds the first crossover potential of the secondary electron emission curve, those various portions of the storage layer impinged by the primary electrons of the electron beam will emit more secondary electrons than the primary electrons received, and hence, will charge up positively. As a result of such emission, there is produced, or written, on the storage layer 34 a bistable electrostatic charge image, or pattern, (not shown) within which there are variations in the level of electrostatic charge potential, (i.e., zero volts or a certain positive voltage) the electrostatic charge potential distribution being a function of the input signals provided to the control electrode 18. The level of electrostatic charge potential at those portions of the storage layer where no information is written is zero volts. The level of electrostatic charge potential at the various portions (not shown) of the storage layer that have information written thereon preferably is between the first and second crossover potentials of the material comprising the storage layer and substantially equal to the particular voltage required for electrical breakdown to occur between the exposed surface 37 of the storage layer 34 and the conductive substrate 30 (which voltage varies with the thickness of the storage layer). This electrical breakdown condition allows the flow of electrical charge carriers (electrons) from the substrate 30 into the storage layer 34 after secondary electrons are emitted by the storage layer 34, as explained below. This flow of charge carriers provides electrical signals that are extracted and then utilized to read out the information in the storage layer 34. The operation of the device described herein does not require, for the operation thereof, any penetration of the storage layer by the beam electrons or any type of bombardment-inducedconductivity. It requires only the capability for electrical breakdown brought about by a sufficient potential charge at a surface of the storage layer.

' In the read operation, a reading beam is deflected in raster fashion across the storage target 20 that exhibits, at the aforementioned portions thereof, electrostatic charge potentials that are preferably both equal to the breakdown voltage for the particular storage layer and between the first and second crossover potentials. Where a particular portion of the storage layer 34 exhibits an electrostatic charge potential that equals the breakdown potential and falls within the potential range between the first and second crossover potentials ofthe material comprising the storage layer 34, the reading beam impinging upon that particular portion will cause the emission therefrom of secondary electrons, which secondary electrons are attracted to the collector electrode 48. Generally, at those other portions, if any, of the storage layer 34 where the charge potential is zero, substantially all of the reading beam will be unable to impinge thereon but will be attracted to the collector electrode 48.

Because the electrostatic charge potential provided on the particular portions of the storage layer 34 are, as previously mentioned, equal to the electrical breakdown voltage of the storage layer, the excess emission of secondary electrons therefrom (i.e., more secondary electronsbeing emitted than primary electrons impinging) upon infringement by the reading beam, results in the flow of electrons from the substrate 30 into these certain portions of the storage layer 34 to offset the change in potential resulting from such secondary electron emission. The flow of electrons from the substrate 30 into the storage layer 34 provides an electrical output signal that can be used with suitable apparatus (e.g., 44 of FIG. 1) known to the art such that a readout of the stored image, or pattern, is obtained.

In another embodiment of the invention, shown in FIG. 3, a storage tube of the type shown in FIG. 1 employs a storage target 70 having storage regions in the form of a storage layer 72 comprising a plurality of discrete bodies in the form of substantially parallel, spaced strips 74, each strip -74 comprising a suitable insulating material. The storage layer 72 is disposed on major surface 76 of a semiconductor substrate 78. In another embodiment of the invention, shown in FIG. 4, a storage tube of the type shown in FIG. 1 employs a storage, target 80 comprising a storage layer 82 dis posed on a major surface of an electrically conducting substrate 86, the storage layer 82 comprising an apertured layer-consisting of a suitable insulating material. In both embodiments (FIGS. 3 and 4), the storage layers 72 and 82 cover only a portion of the respective major surfaces 76 and 84, other portions of the major surfaces 76 and 84 being exposed. The storage target shown in FIG. 3 is disposed such that the strips 74 face the electron gun. The scanning direction of the beam 62 is preferably substantially perpendicular to the major axes of the strips 74.

In the embodiments shown in FIGS. 2, 3, and 4, the substrates (30, 78, and 86, respectively) may, as previously stated, consist essentially of a semiconductor material (for example, silicon or germanium having an n type, ntype or n+ type, or p type, p type or p+ type conductivity) and the storage layers (34, 74, and 82, respectively) may consist essentially of an insulating compound (e.g., the dioxide of nitride) of the type of semiconductor material of which the substrate consists. The storage target may be made by techniques known in the art. For example, the storage layer 34 (FIG. 2) may be produced by, first, providing on the substrate 30 a continuous layer (not shown) of an insulating compound of the semiconductor material used for the substrate 30. Where the semiconductor material of the substrate is silicon and the insulating compound thereof is silicon dioxide, the above-mentioned continuous layer of insulating compound may be produced, for example, by one of the well-known techniques of thermal oxidation of the substrate in steam; chemical vapor deposition from a mixture of silane and oxygen; or anodic oxidation. After the continuous insulating layer is deposited, mutually substantially perpendicular spaces 38 are produced therein by photoresist methods known in the art. The spaces 38 extend completely across the layer 34. The storage target 20 thus fabricated may now be mounted on suitable support means, such as a target support ring 46 (FIG. 1), by an adhesive or in some other suitable manner. Thereafter, the mounted target is disposed within the envelope 12 (FIG. 1) as by bonding the target support ring 46 to the envelope walls by means of adhesive, for example.

In accordance with the present invention, the thickness of the respective storage layer, 34, 74, or 82, of the storage target is adjusted such that electrical breakdown between the electron beam-accessible surface of the storage layer an the underlying substrate, can be attained at some breakdown potential value between the first and second cross-over potentials of the particular material comprising the storage layer. At those po tentials greater than the breakdown potential of the storage layer, the layer acts substantially as a conductor, whereas the layer acts substantially as an insulator at those potentials less than the breakdown potential.

FIGS. 5 through 11 schematically illustrate the stor age target 20 of FIG. 1 at various phases (viz., write, read, and erase) of one mode of operating the storage tube 10 of FIG. 1. This mode includes writing on the storage target 20 by causing excess emission of secondary electrons from the storage layer 34 of the target 20. The storage target 20 shown in FIGS. 5 through 11 comprises both an electrically conductive or semiconductive substrate 30 and a storage layer 34, the storage layer 34 comprising for purposes of illustration, spaced insulating (or storage) portions A, B, C, and D. Illustrative charge potential values for the respective insulating regions are provided thereabove in FIGS. 5 through 11. In FIGS. 5 through 11, all potentials (V applied to the semiconductor substrate are in volts with respect to the potential of the cathode 16. A collector electrode 48 is also shown in these figures.

In FIGS. 5 through 11, for purposes of illustration, the storage layer is of silicon dioxide and has a thickness (e.g., about 1,000 A) that will allow electrical breakdown therein at an electrostatic charge potential (e.g., about +60V.) between the first and second crossover potentials for the material comprising the storage layer. Also, the cathode potential is considered to be at ground in these figuresv Referring now to FIG. 5, with no electron beam directed toward the target 20 and no potential applied thereto, the storage layer potential is zero volts.

In FIG. 6, there is first applied to the substrate 30 a voltage that is sufficient to raise the charge potential on the storage layer 34 to a level exceeding the first crossover potential (but below the second crossover potential) on the secondary emission curve for the particular insulating material used for the storage layer 34. For charge potentials exceeding the first crossover potential, the secondary electron emission ratio will exceed unity. For a silicon dioxide layer, a potential of +lOOV. with respect to cathode potential exceeds the first cross-over potential but is below the second cross-over potential thereof. This applied potential of +lOOV. brings about a charge of +lOOV. at the top surface 37 of the storage layer 34, this change is potential of the storage layer 34 being brought about by capacitive coupling between the substrate majorsurface 32 on which the storage layer 34 is disposed and the top surface 37 of the storage layer 34.

Referring now to FIG. 7, the electron gun 14 is turned on and a high velocity writing beam 62a is caused to scan the target 20 while the substrate potential is maintained at +lOOV. (i.e., at a potential between the first and second cross-over voltages). Because the storage layer 34 has a potential (i.e., +1 V.) exceeding the first cross-over potential value of the material comprising it, secondary electrons are emitted, at a secondary-to-primary ratio greater than one, from the storage layer 34 as that layer is scanned by the beam 62a. The number of secondary electrons that is emitted is dependent upon the beam current which is modulated, as stated before, by the input signal to the control electrode 18 of the electron gun 14.

Because of the modulation of the electron beam current by the input signal, some portions (i.e., A, B, and D) but not others (C) of the storage layer 34 that are impinged by the beam exhibit, as shown in FIG. 7, an increase in the charge potential thereon to a level (e.g., +l60V.) that exceeds the applied potential (V,) by an amount equal to the particular breakdown voltage for the storage layer. This increase in potential is due to more secondary electrons leaving the storage layer 34 at these portions A, B, and D, than primary electrons of'the beam 62a arriving thereat. That is, in this particular case, as the target is scanned the input signal so modulates the beam current that quantities of second ary electrons sufficient to provide distinguishable increases in charge potential are emitted by the insulating portions A, B, and D impinged by the writing beam 62a, but not by the other portion C. The secondary electrons are collected by the collector electrode 48, which is maintained at a potential (for example, +500V. with respect to the cathode potential) that is significantly higher than the potentials at the storage target such that emitted secondary electrons are attracted thereto. The charge potential on the storage layer 34 is substantially limited to a value of +160V. since electrical breakdown conditions exist (i.e., the 60V. potential difference between the surface 37 and the substrate equals the breakdown potential) and any attempt to exceed this value (i.e., +160) by secondary electron emission causes electrons to flow into the storage layer, thereby preventing a potential exceeding +l6OV.

Thereafter, the beam is turned off and the applied substrate potential (V,) is reduced to a value substantially equal to the cathode potential (i.e., to zero volts with respect to the cathode) as shown in FIG. 8. Because of capacitive coupling, the charge potentials on the storage layer 34, are, upon reduction of the applied substrate potential (V,) reduced by a corresponding value (i.e., 100V.) Consequently, the charge potentials on particular portions A, B, and D of the storage layer 34 are between the first and second cross-over voltages (i.e., at about +60V.). That portion (i.e., C) of the storage layer where writing has'not occured does not exhibit an electrostatic charge potential but has a potential of zero volts with respect to the cathode. The level of electrostatic potential at the portions A, B, and D of the storage layer 34 should, in addition to being at a value between the first and second cross-over potentials be sufficient to cause electrical breakdown through the storage layer 34. For a storage layer consisting essentially of silicon dioxide and having a thickness of about 1,000 A, a potential, at the surface (e.g., 37) of the storage layer, of about +6OV. with respect to the conductive or semiconductive substrate is sufficient to produce such electrical breakdown. At this stage, writing has been completed and the information stored in the target 20 in the form of a charge pattern can be read.

In the reading operation shown in FIG. 9, while the applied substrate potential (V is maintained at zero volts the electron gun 14 is turned on to provide a'reading beam 62b which scans the target 20 in raster fashion. Because the charge potential at certain areas exceeds the first cross-over voltage, secondary electrons (shown, for simplicity, as being emitted from only one portion of the storage layer 34) will be emitted from the areas of the storage layer where information has been written, these secondary electrons being attracted to the collector electrode 48. Substantially, none of the electrons of the reading beam 62b land on the unwritten areas of the storage layer or on the exposed portions of the substrate major surface 32 because the potential (i.e., zero volts with respect to the cathode) is not sufficient to create an electric field capable of attracting these electrons. Substantially all of beam electrons that are not attracted to the positively charged portions (i.e., those where information is written) of the storage layer are attracted to the collector electrode 48. Because the potentials at the abovementioned certain portions of the storage layer are sufficient to cause electrical breakdown therethrough, electrical charge carriers pass from the substrate into the storage layer, the quantity of such charge passing into the storage layer being comparable to the quantity of secondary electrons emitted from the storage layer. The flow of such electrical charge carriers into the storage layer provides an output signal in the form of a variation in the current applied to the signal plate (i.e., substrate 30), which signal is extracted via the substrate and transmitted to a remote display tube 44 for visual display, or utilized in some other manner. Reading in the above mode is done non-destructively. Because the levels of electrostatic charge potential on the storage layer are +60 and zero volts, the charge pattern is bistable, as is the read-out of the stored information. Thus, a visual display of the stored image would be in black and white (i.e., a bistable display).

Referring now to FIG. 10, when it is desired to erase the stored information, the substrate potential (V,) is not changed, but the potential applied to the collector electrode 48 is reduced to a relatively low level (about +20V) and an erasing beam 620 is produced, the beam 62c providing electrons which will land upon the storage layer 34, which storage layer 34 is positively charged (as shown in FIG. Such landing of electrons causes the storage layer 34 to charge down to a zero charge potential with respect to cathode potential, as shown in FIG. I 1. The storage target is now available for information storage, and the above writing and reading steps may be repeated.

In certain instances (FIG. 12) after the completion of the writing operation but before the reading operation, the respective levels of electrostatic charge potential at the various portions (e.g., B, C, and D) of the storage layer 34 where information is intended to be written, deviate slightly from the abovementioned preferred written condition, in which preferred condition the charge potential at these written portions equals the breakdown voltage and is between the first and second cross-over potentials, and these levels of charge poten- .tial at the other portions (e.g., A) where no information is intended to be written, deviate slightly from the above-mentioned preferred unwritten condition where the charge potential at the unwritten portions are'at zero volts with respect to the cathode potential. Such deviations in potential can be caused, for example, by undesirable variations in the writing beam current. As shown in FIG. 12 (where, for example, the storage layer 34 comprises silicon dioxide and has a breakdown voltage of about +6OV. and a first crossover potential of about +3OV.) the various charge potentials can be below the first cross-over potential and less than the electrical breakdown voltage, this being the situation at portion A which was intended to be an unwritten portion; between the first and second crossover potentials but less than the breakdown voltage, this being the situation at region C, which is intended to be a written portion, or greater than the first crossover potential and equal to the breakdown voltage, this being the situation at portions Band D, which are written regions. The device can, in this instance, be operated in the following way to read out, in bistable form, the stored information.

A reading beam (e.g., 62b in FIG. 10) is caused to scan across the target 20 shown in FIG. 12. At portion A, where the charge potential (i.e., +5 V.) is less than both the breakdown voltage and the first cross-over potential, electrons from the reading beam land, at the beginning of the reading operation, and charge the potential thereat down to zero volts, as shown in FIG. 13. As the reading operation continues, no excess secondary electrons are emitted from portion A and no breakdown occurs there so that no read-out signal is provided by portion A, the result being that portion A acts as an unwritten portion, which was the result originally sought.

At portions B and D, the stored information will be read out as describedabove. At storage layer portion C, where the charge potential (i.e., +55V) exceeds the first cross-over potential but is' less than the desired breakdown voltage, the electrons from the reading beam impinging thereon initially cause the excess emission of secondary electrons therefrom. As a result, there is a net loss of electrons, this loss continuing for a relatively short time until the charge potential at portion C reaches the break-down voltage (i.e., +6OV), as shown in FIG. 13, after which the information stored at portion C is read out in the above described manner (i.e., secondary electron emission, etc.). The changes in electrostatic charge potential at portions A and C can take place in a small number of frames, so that the information can be read out with no significant impairment of the information. The levels of electrostatic charge potential (i.e., zero at the unwritten portions, and a potential value that is between the first and second cross-over potentials and equal to the breakdown voltage thereof at the written portions) on the storage target remain substantially unchanged for comparatively long periods of time, even when the information stored in the target is read out continuously or for intermittent extended periods. This is because the written portions of the storage layer are not able to sustain potentials exceeding the breakdown voltage so that any excess charges thereat and conducted to the signal plate. The potentials at these'written portions are not able to exhibit, for any significant period of time, a potential below the breakdown voltage, this being so because a collector electrode voltage greater than the breakdown causes excess secondary electrons to'be emitted (due to the bombardment of the target by the reading beam) from the stroage layer, which secondary electron emission raises the potential of the storage layer to the breakdown voltage. At the unwritten portions, any increase in potential there above zero volts causes electrons from the reading beam to land thereon, thereby offsetting any such increase in potential and restoring the potential to zero volts.

Among the advantages of the present invention are the ability to read out stored information for relatively long periods of time with substantially no destruction of the information and relatively fast write, erase, and read operations.

Further advantages are relative ease of manufacturing of the novel device and relatively low cost.

I claim:

1. An electron device having information storage capability, comprising:

a. an evacuated envelope;

b. a storage target disposed within said envelope and comprising storage regions and electrically conductive or semiconducting regions, said storage regions comprising a layer of an electrically insulating material characterized by a secondary electronemission ratio exceeding unity over a certain potential range and having a thickness causing electrical breakdown therethrough at a preselected potential within said certain potential range;

means for applying to and extracting from said conducting regions electrical potentials;

d. means for directing an electron beam toward said storage regions to give rise to said preselected potential at certain ones of said storage regions; and

e. means for collecting secondary electrons emitted from said target.

2. An electron device having information storage capability, comprising:

a. an evacuated envelope;

ill

b. a target electrode within said envelope including a layer of electrically insulating material and a signal plate disposed in contact with a surface of said layer, said layer being of such thickness causing electrical breakdown thereof at a certain potential difference thereacross, said potential being between the first and second crossover potentials of said insulating material;

c. connection means for applying certain electrical signals to and extracting electrical potentials from said signal plate;

. writing means responsive to an input signal for producing a charge pattern on said layer, said pattern comprising either and only zero volts potential or said certain potential;

e. reading means for extracting stored information from said target;

f. means for erasing information stored on said target;

g. a collector electrode disposed in spaced relation with said target electrode; and

h. means for applying an electrical potential to said collector electrode.

3. An electron device having information storage capability, comprising;

a. an evacuated envelope;

b. a storage layer within said envelope comprising an electrically insulating material having a secondaryto-primary electron emission ratio greater than one at a certain electrical potential, said layer having a thickness causing electrical breakdown therethrough at substantially said certain electrical potential;

0. means for causing excess secondary electrons to be emitted from said storage layer, said secondary electrons being greater innumber than primary electrons impinging on said storage layer for charging said storage layer to said certain potential;

d. means for collecting said secondary electrons; and

e. means for providing electrical charge carries to said layer upon said electrical breakdown, said charge carriers being substantially equal in number to the emitted said secondary electrons.

4. A method of operating an information storage device including i. a storage target disposed within said envelope comprising an electrically conductive or semiconductive substrate, and a storage layer disposed on a surface of said substrate, said storage layer comprising an electrically insulating material exhibiting a secondary-to-primary electron emission ratio greater than unity over a certain potential range, said storage layer having a thickness causing electrical break down thereof at a potential within said certain potential range;

ii. means for collecting secondary electrons emitted from said target;

iii. means for applying to and extracting from said substrate various electrical potentials; and

iv. means for scanning said storage layer with an electron beam, said method comprising:

a. producing an electrostatic charge pattern on said storage layer, the electrostatic charge potential at at least one portion of said storage layer being substantially equal to said one potential, so that an electrical breakdown condition exists at said at least one portron;

b. scanning said storage layer with said electron beam so as to cause secondary electron emission from said at least one portion of said storage layer, the number of emitted secondary electrons exceeding the number of primary electrons impinging upon said at least one portion of said storage layer;

0. collecting said secondary electrons with said collecting means;

(1. supplying electrons to said at least one portion from said substrate in numbers substantially equal to said emitted secondary electrons, said supplied electrons providing an output signal; and

e. extracting said output signal.

5. An electrical device having an information storing capability comprising:

a. an information storage structure comprising storage regions and electrically conducting or semiconducting regions, said storage regions comprising a layer of insulating material of such thickness that electrical breakdown therethrough occurs at an electrical potential at which the secondaryemission ratio of said material exceeds unity;

b. means for accumulating charge on said storage region in correspondence with information to be stored to give rise to said breakdown electrical potential across certain ones of said storage means; and

c. means for detecting the presence of said breakdown electrical potential via said electrical conducting or semiconducting regions.

6. An electrical device as in claim 5 wherein said detecting means comprises means for inducing secondary emission of electrons from said ones of said storage means, whereby current is caused to flow through said one regions by virtue of electrical breakdown thereof. =u= 

1. An electron device having information storage capability, comprising: a. an evacuated envelope; b. a storage target disposed within said envelOpe and comprising storage regions and electrically conductive or semiconducting regions, said storage regions comprising a layer of an electrically insulating material characterized by a secondary electron-emission ratio exceeding unity over a certain potential range and having a thickness causing electrical breakdown therethrough at a preselected potential within said certain potential range; c. means for applying to and extracting from said conducting regions electrical potentials; d. means for directing an electron beam toward said storage regions to give rise to said preselected potential at certain ones of said storage regions; and e. means for collecting secondary electrons emitted from said target.
 2. An electron device having information storage capability, comprising: a. an evacuated envelope; b. a target electrode within said envelope including a layer of electrically insulating material and a signal plate disposed in contact with a surface of said layer, said layer being of such thickness causing electrical breakdown thereof at a certain potential difference thereacross, said potential being between the first and second crossover potentials of said insulating material; c. connection means for applying certain electrical signals to and extracting electrical potentials from said signal plate; d. writing means responsive to an input signal for producing a charge pattern on said layer, said pattern comprising either and only zero volts potential or said certain potential; e. reading means for extracting stored information from said target; f. means for erasing information stored on said target; g. a collector electrode disposed in spaced relation with said target electrode; and h. means for applying an electrical potential to said collector electrode.
 3. An electron device having information storage capability, comprising: a. an evacuated envelope; b. a storage layer within said envelope comprising an electrically insulating material having a secondary-to-primary electron emission ratio greater than one at a certain electrical potential, said layer having a thickness causing electrical breakdown therethrough at substantially said certain electrical potential; c. means for causing excess secondary electrons to be emitted from said storage layer, said secondary electrons being greater in number than primary electrons impinging on said storage layer for charging said storage layer to said certain potential; d. means for collecting said secondary electrons; and e. means for providing electrical charge carries to said layer upon said electrical breakdown, said charge carriers being substantially equal in number to the emitted said secondary electrons.
 4. A method of operating an information storage device including i. a storage target disposed within said envelope comprising an electrically conductive or semiconductive substrate, and a storage layer disposed on a surface of said substrate, said storage layer comprising an electrically insulating material exhibiting a secondary-to-primary electron emission ratio greater than unity over a certain potential range, said storage layer having a thickness causing electrical break down thereof at a potential within said certain potential range; ii. means for collecting secondary electrons emitted from said target; iii. means for applying to and extracting from said substrate various electrical potentials; and iv. means for scanning said storage layer with an electron beam, said method comprising: a. producing an electrostatic charge pattern on said storage layer, the electrostatic charge potential at at least one portion of said storage layer being substantially equal to said one potential, so that an electrical breakdown condition exists at said at least one portion; b. scanning said storage layer with said electron beam so as to cause secondary electron emission from said at least one portion of said storaGe layer, the number of emitted secondary electrons exceeding the number of primary electrons impinging upon said at least one portion of said storage layer; c. collecting said secondary electrons with said collecting means; d. supplying electrons to said at least one portion from said substrate in numbers substantially equal to said emitted secondary electrons, said supplied electrons providing an output signal; and e. extracting said output signal.
 5. An electrical device having an information storing capability comprising: a. an information storage structure comprising storage regions and electrically conducting or semiconducting regions, said storage regions comprising a layer of insulating material of such thickness that electrical breakdown therethrough occurs at an electrical potential at which the secondary-emission ratio of said material exceeds unity; b. means for accumulating charge on said storage region in correspondence with information to be stored to give rise to said breakdown electrical potential across certain ones of said storage means; and c. means for detecting the presence of said breakdown electrical potential via said electrical conducting or semiconducting regions.
 6. An electrical device as in claim 5 wherein said detecting means comprises means for inducing secondary emission of electrons from said ones of said storage means, whereby current is caused to flow through said one regions by virtue of electrical breakdown thereof. 